The present invention relates to a radio frequency or optical communication system in which a relay station is used to aid communication between two parties, and more particularly to an improvement allowing more efficient use of the available channel resource.
Radio frequency or optical communication between two physically separate devices using a third party relay station is illustrated in FIG. 1. The relay station receives a signal from each of the two user devices and retransmits the composite received signal back to both of the users, typically on a frequency channel offset from the receiving frequency channel. Two examples are:
1. Satellite communication between two earth terminals (ground, airborne, or shipboard) using a non-processing "bent pipe" transponder in a linear mode. PA1 2. Terrestrial line-of-sight radio communication between two terminals, using a relay radio to connect them (such as in a tethered aircraft or a radio tower). PA1 The roundtrip propagation delay to and from the relay station. PA1 Relative signal amplitude, carrier frequency and phase of the relayed source signal vs. the a priori known source signal.
The relay station may simultaneously retransmit many more signals than the two shown in FIG. 1. The relay resource is typically shared using any of the well known multiple access techniques such as Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), Code Division Multiple Access (CDMA) or hybrids which use one or more of these techniques in combination. In each of these techniques, the relay resource is effectively channelized, so that the receiver can, by tuning in on a specific frequency, at a specific time, with a specific code, or some combination thereof, isolate a particular received signal from all the others. For simplicity, the following discussion will be based on the FDMA technique, although the apparatus may be used with TDMA, CDMA, or hybrid multiple access techniques.
FIG. 2 shows a pair of signals separated in the frequency domain between User 1 and User 2. The signal transmitted by User 1 has a center frequency of f.sub.1 Hz and a bandwidth of W.sub.1 Hz. The signal transmitted by User 2 has a center frequency of f.sub.2 Hz and has a bandwidth of W.sub.2 Hz. With the FDMA approach, the total bandwidth required to support simultaneous transmissions from the pair of users is (W.sub.1 +W.sub.2) Hz. The other signals being relayed are assigned to other non-overlapping bandwidths. Thus the number of users is limited by the bandwidth of the relay. Although this is a straightforward partitioning of the relay resource among a number of users, it is not necessarily the most efficient use.
In telephone communications, because of the limited bandwidth of the twisted-pair circuits, there is of necessity a more efficient use of the available bandwidth, in which the transmit and receive signals share the same bandwidth. However this type of communication is not relayed, in the sense of the word used herein, but rather it is point-to-point, although the signal may be routed through several intermediary nodes on its way from the source to the destination. Because the transmit and receive signals occupy the same bandwidth, devices known as hybrids are used to separate the combined signals on the two wires into the transmit and receive portions. However these devices, which are located at the two telephone company central offices, do not perfectly separate the signals, so an unwanted consequence is echoes. The received signal will thus contain two echoes of the transmitted signal, a near echo from the local central office, and a far echo from the remote central office.
For data communications, these echoes are removed by an echo canceller, which subtracts a properly scaled and delayed replica of the transmitted signal from the received signal. Because the signal is not relayed and there no frequency offset between the transmit and receive signals, the echo canceller can be implemented using an adaptive filter approach. The coefficients of the filter can be estimated using any one of a number of well-known algorithms such as the LMS (least-mean-square) algorithm. However, this approach cannot be extended to the case of a relayed communication system in which the transmitting and receiving frequencies are significantly different. Thus, a different cancellation technique must be employed in order for relayed signals to be able to share the same bandwidth.
What is needed is a mechanism to provide more efficient use of the available channel resource to support two-way relayed communication for any pair-wise connection among a group of radio frequency or optical terminals. More specifically, what is needed is an improvement which is independent of the multiple access technique in use, independent of the modulation and coding of the system and which is without the benefit of any data compression techniques or need for special equipment at the relay station.